1. Field of the Invention
The present invention relates to a refractory model material for making dental porcelain restoratives by a building-up method. More specifically, the present invention concerns a dental refractory model material usable for dental porcelain restoratives such as porcelain laminated veneers, porcelain inlays or onlays and porcelain jacket crowns by taking an impression of formed teeth in the mouth with the use of a dental impression material, casting a blend of a powdery component with a colloidal silica dispersion into the impression as a slurry slush, which is then cured to prepare a refractory model, and building a finely divided porcelain slush directly upon the model, which is then placed in a furnace, etc. in its entirety to heat and burn the porcelain.
2. Statement of the Prior Art
Heretofore, refractory model materials based on phosphates and gypsum have been used as the refractory model materials for preparing dental porcelain restoratives such as porcelain laminated veneers. Soluble phosphates and magnesium oxide or gypsum have been used as the binding component therefor, and crystalline quartz and crystobalite as the aggregate component therefor. For the phosphate base refractory model materials, a colloidal silica dispersion has been used as a liquid component.
However, such conventional refractory model materials have had the following defects.
(1) Crystalline quartz and crystobalite incorporated as the main component of an aggregate undergo a sharp volume change due to crystal transformation in the vicinity of 573.degree. C. and 200.degree.-300.degree. C., respectively. With the conventional refractory model materials, it has thus been pointed out that since it is impossible to control sharp changes in the thermal expansion of crystalline quartz and crystobalite, crazing or cracking problems arise, when models are repeatedly burned.
(2) Sharp changes in the volume of the refractory model materials give rise to the distortion of built-up porcelain during burning, thus causing the porcelain to be cracked.
(3) Even though the mixing ratio of crystalline quartz and crystobalite used as the main component of the aggregate is varied, the conventional refractory model materials show only a slight variation in the rate of changes in heating. Thus, it is difficult to accommodate them to various types of commercially available porcelain having varied rates of changes in heating.
(4) When models are prepared and removed from impression materials, it is most likely that they may be fractured due to a low green strength of the refractory model materials.
(5) When a finely divided porcelain slush is condensed during building-up, it is most likely that the surfaces of models may be damaged by a condensing instrument by reason of a strength after burning with nothing placed thereon (hereinafter referred to as a post-burning strength) being low. The conventional model materials are also short of reasonable durability.
(6) In some cases, the conventional model materials may cause surface roughening of impressions, failing to provide smooth model surfaces. Thus, some difficulty is involved in precise dental manipulations.